An adjustable high chair comprises a frame, a seat portion, and a backrest sequentially connected with each other, as well as a tray connected to the top end of the seat portion, in addition to an adjusting unit installed at the back of the backrest. The adjusting unit comprises a guiding piece for guiding movement, and an actuating piece, at least a transmitting piece, a sliding piece, and a securing piece sequentially cooperated each other, as well as a button served as a secondary safety lock. Two distal ends of the securing pieces are respectively engaged into a plurality of recesses to fix the backrest at a certain inclination angle. When the actuating piece is pressed to the extent that the securing piece is escaped from the recesses, the backrest can be adjusted and its inclination can be changed. If the actuating piece and the button are pressed simultaneously, the backrest can be adjusted to have the largest inclination angle. Besides, the seat portion comprises a movable piece having bumps which are engaged into slits formed at the arms in different heights to adjust the tray in height.